1. Field of the Invention
The present invention relates to an optical disk having a plurality of reflection layers having pits or grooves carrying information, formed into a multi-layer structure.
2. Description of Related Art
Multi-layer optical disks are known as disks having a structure that a plurality of reflection layers respectively having pits or grooves carrying information are formed into a multi-layered structure, so that the information carried by each layer is optically read from one side of the disk. Among various multi-layer optical disks, double layer optical disks are used widely.
In recent years, there is a demand for increasing the recording density of each reflection layer of the multi-layer optical disk. To meet the requirement of high-density recording of information, it is conceivable to make pits or grooves on each reflection layer smaller and form them at higher density, to use a laser beam of a shorter wavelength for reading information carried by such a high-density disk.
When a gold thin film is used for the reflection layer, however, the reading of information on the reflection layer by means of the laser beam of a shorter wavelength becomes inaccurate because the optical characteristics of the gold thin film is largely wavelength dependent. Thus, when gold is used, the first layer of the double-layer optical disk becomes unable to function as an intermediate layer.
If, on the other hand, a dielectric multilayer is used as the intermediate reflection layer, it is possible to obtain desired optical characteristics. However, the use of dielectric multilayer has been practical not possible because of the problems such that the available percentage in its production process is very low because a number of steps are required to produce the layer whose total thickness amounts to more than one thousand nanometers, that desired reflectivity and transmittivity cannot be stably obtained because of the generation of cracks during its production process, and that degrading of the signal is caused by changes of the effective pit shape or optical interference.